Overview.bs

<pre class="metadata">
Title: CSS Multi-column Layout Module Level 2
Group: CSSWG
Shortname: css-multicol
Level: 2
Status: FPWD
Prepare for TR: yes
Date: 2024-12-19
Work Status: Exploring
ED: https://drafts.csswg.org/css-multicol-2/
TR: https://www.w3.org/TR/css-multicol-2/
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Editor: Rachel Andrew, Google, rachelandrew@google.com, w3cid 81117
Former Editor: Håkon Wium Lie, Opera Software, howcome@opera.com, w3cid 9796
Abstract: This specification describes multi-column layouts in CSS, a style sheet language for the web. Using functionality described in the specification, content can be flowed into multiple columns with a gap and a rule between them.
Link Defaults: css-color (property) color, css-sizing-3 (property) max-height, css-backgrounds-3 (value) hidden
Include Can I Use Panels: yes
WPT Path Prefix: /css/css-multicol/
</pre>

<pre class="ignored-specs">
spec: css21;
</pre>
<pre class=link-defaults>
spec:css-break-4; type:dfn; text:fragment
</pre>

<style type="text/css">
.cols { width: 500px; height: 200px; background: #fff; position: relative; border: solid 5px blue; margin: 0.5em 2em 1em 0; font: bold 14px/19px Arial, sans-serif }
.cols p { padding: 3px; margin: 0 }
.col { position: absolute; left: 0px; top: 0; z-index: 6; width: 170px }
.gap { position: absolute; background: green; width: 5px; bottom: 0px; top: 0px; border: 10px solid yellow; border-top-width: 0; border-bottom-width: 0;  }
.rep { position: absolute; top: 45px; background: black; height: 110px; width: 100px;  color: white; z-index: 4 }

table.breaks { border-collapse: collapse; margin: 1em 0 }
table.breaks td, table.breaks th { border: thin solid black; padding: 0.1em 0.2em }

div.example:before { width: 9em }
</style>

<h2 id="introduction">
Introduction</h2>

(This section is not normative.)

	This module describes <dfn export>multi-column layout</dfn> in CSS.
	By using functionality described in this document,
	style sheets can declare that the content of an element
	is to be laid out in multiple columns.

	Other layout methods in CSS,
	when applied to a parent element,
	change the display properties of the direct children.
	For example if a three column grid layout is created,
	the direct children of the [=grid container=] become [=grid items=]
	and are placed into the column tracks,
	one element per cell with additional rows created as needed.

	The child elements of a [=multi-column container=] however continue in normal flow,
	that flow is arranged into a number of columns.
	These columns have a flexible inline size,
	and therefore respond to available space
	by changing the size or number of columns displayed.

	Multi-column layouts are easy to describe in CSS.
	Here is a simple example:

	<div class=example>
		<pre highlight="css">body { column-width: 12em }</pre>

		In this example, the <code class=html>body</code> element is
		set to have columns at least ''12em'' wide. The exact number of
		columns will depend on the available space.
	</div>

	The number of columns can also be set explicitly in the style sheet:

	<div class=example>
		<pre highlight="css">body { column-count: 2 }</pre>

		In this case, the number of columns is fixed
		and the column widths will vary depending on the available width.
	</div>

	The shorthand 'columns' property can be used to set either,
	or both, properties in one declaration.

	<div class=example>
		In these examples, the number of columns, the width of columns, and
		both the number and width are set, respectively:

		<pre highlight="css">
		body { columns: 2 }
		body { columns: 12em }
		body { columns: 2 12em }
		</pre>

	</div>

	Another group of properties introduced in this module describe
	gaps and rules between columns.

	<div class=example>
		<pre highlight="css">
		body {
			column-gap: 1em;
			column-rule: thin solid black;
		}
		</pre>

		The first declaration in the example above sets the gap
		between two adjacent columns to be 1em.
		Column gaps are similar to padding areas.
		In the middle of the gap there will be a rule
		which is described by the 'column-rule' property.
	</div>

	The values of the 'column-rule' property are similar to those of the CSS 'border' properties.
	Like 'border', 'column-rule' is a shorthand property.

	<div class=example>
		In this example, the shorthand 'column-rule' declaration from the
		above example has been expanded:

		<pre highlight="css">
		body {
			column-gap: 1em;
			column-rule-width: thin;
			column-rule-style: solid;
			column-rule-color: black;
		}
		</pre>
	</div>

	The 'column-fill' and 'column-span' properties
	give style sheets a wider range of visual expressions in multi-column layouts.

	<div class=example>
		In this example, columns are set to be balanced,
		i.e., to have approximately the same length.
		Also, <code>h2</code> elements are set to span across all columns.

		<pre highlight="css">
		div { column-fill: balance }
		h2 { column-span: all }
		</pre>
	</div>

	<wpt>
		multicol-fill-balance-029.html
	</wpt>

	This specification introduces ten new properties,
	all of which are used in the examples above.

	If all column properties have their initial value,
	the layout of an element will be identical
	to a multi-column layout with only one column.

	<div class="example">
		<a>Column gaps</a> (diagonal hatching) and <a>column rules</a>
		are shown in this sample rendition of a multi-column container
		with padding (cross hatching).
		The hatched areas are present for illustrational purposes only.
		In actual implementations these areas will be determined by the background,
		the second image shows a rendering of a [=multi-column container=] with column-rules.

		<figure>
			<img alt="a diagram showing the various parts of multi-column layout" src="images/initial-example.svg" no-autosize>
			<img alt="key to the conventions used to display invisible parts of diagram" src="images/invisible-elements.svg" no-autosize>
			<figcaption>A multi-column layout with the non-visible column-span and padding inside the multicol container highlighted.</figcaption>
		</figure>

		<figure>
			<img alt="a diagram showing the various parts of multi-column layout" src="images/initial-example-b.svg" no-autosize>
			<figcaption>The same layout as in the first image, as it would be displayed by an implementation.</figcaption>
		</figure>
	</div>

<h3 id="values">
Value Definitions</h3>

	This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS21]]
	using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
	Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
	Combination with other CSS modules may expand the definitions of these value types.

	In addition to the property-specific values listed in their definitions,
	all properties defined in this specification
	also accept the <a>CSS-wide keywords</a> as their property value.
	For readability they have not been repeated explicitly.

<h2 id="interaction">Module Interactions</h2>

This document defines new features not present in earlier specifications.
In addition, once final, it will replace and supersede the following:

<ul>
<li>The <a href="https://www.w3.org/TR/css3-multicol">CSS Multi-Column Layout Module Level 1</a>
</ul>

<h2 id="the-multi-column-model">
The multi-column model</h2>

	An element whose 'column-width' or 'column-count' property is not ''column-width/auto''
	establishes a <dfn lt="multi-column container|multicol container" oldids="multi-column-element" export>multi-column container</dfn>
	(or <i>multicol container</i> for short),
	and therefore acts as a container for [=multi-column layout=].

	<wpt title="Basic multicol tests.">
	multicol-basic-002.html
	multicol-basic-004.html
	</wpt>

	<wpt title="Tests demonstrating that auto values do not create a multicol container.">
	multicol-count-computed-004.xht
	</wpt>

	<wpt title="Multicol properties do not inherit.">
	inheritance.html
	</wpt>

	<wpt title="Multicol with scrolled columns.">
	multicol-scroll-content.html
	</wpt>

	<wpt title="Multicol with zero height.">
	multicol-zero-height-001.xht
	multicol-zero-height-002.html
	multicol-zero-height-003.html
	</wpt>

	In the traditional CSS box model,
	the content of an element is
	flowed into the content box of the corresponding element.
	Multi-column layout introduces a [=fragmentation context=]
	formed of <a lt="anonymous box">anonymous</a> [=fragmentation containers=]
	called <dfn export lt="column box" local-lt=column>column boxes</dfn>
	(or <i>columns</i> for short).
	These [=column boxes=] establish
	an independent [=block formatting context=]
	into which the multi-column container's content flows,
	and form the [=containing block=] for its non-positioned children.

	<div class=example>
		In this example, the width of the image is set with these rules:

		<pre highlight="css">
		img {
			display: block;
			width: 100%;
		}
		</pre>

		Given that the column box creates a new [=block formatting context=],
		the 'width' is calculated relative to the column box.
		Therefore the image will not overflow the column box:

		<figure>
			<img src="images/image-inside-column.svg" alt="an image contained inside a column box" no-autosize>
			<figcaption>The image is constrained by the column box that it is displayed in.</figcaption>
		</figure>
	</div>

	<div class="example">
		Given that the column box creates a new [=block formatting context=], a top margin set on the first child element of a multicol container will not collapse with the margins of the multicol container.

		<figure>
			<img src="images/margins-do-not-collapse.svg" alt="The first paragraph has a 'margin-top' of ''1em'', which appears before the text." no-autosize>
			<figcaption>The margin above the first paragraph has not collapsed, leaving a ''1em'' margin above the first line in the multicol container.</figcaption>
		</figure>
	</div>

	<wpt>
	multicol-margin-001.xht
	multicol-margin-002.xht
	multicol-margin-003.html
	multicol-margin-child-001.xht
	multicol-nested-margin-001.xht
	multicol-nested-margin-002.xht
	multicol-nested-margin-003.xht
	multicol-nested-margin-004.xht
	multicol-nested-margin-005.xht
	multicol-collapsing-001.xht
	</wpt>

	Floats that appear inside multi-column layouts are positioned with
	regard to the [=column box=] where the float appears.

	<div class="example">
		In this example, this CSS fragment describes the presentation of the image:

		<pre highlight="css">
		img {
			display: block;
			float: right;
		}
		</pre>

		In the HTML, the image appears after the sentence ending, "the leg of a chicken".

		<figure>
			<img src="images/image-floated-in-column.svg" alt="an image floated and contained inside a column box" no-autosize>
			<figcaption>The image is floated inside the column box it appears in.</figcaption>
		</figure>
	</div>

	Content overflowing a [=column box=] in the [=block axis=]
	[=fragments=] and continues in the next [=column box=].

	Note: Column boxes, which are [=anonymous boxes=],
	do not become the [=containing block=]
	for [=absolutely positioned boxes=].
	The 'position' property, which establishes a containing block for such boxes,
	applies to the [=multicol container=], it being the [=principal box=].

	<wpt>
	multicol-containing-001.xht
	multicol-containing-002.xht
	multicol-containing-003.html
	multicol-contained-absolute.html
	abspos-autopos-contained-by-viewport-000.html
	abspos-autopos-contained-by-viewport-001.html
	abspos-multicol-in-second-outer-clipped.html
	</wpt>

	<div class="example">
		In this example, the [=multi-column container=] has ''position: relative''
		thus becoming the containing block.
		The image is a direct child of the [=multi-column container=]
		and has ''position: absolute''.
		It takes positioning from the [=multi-column container=]
		and not from the [=column box=].

		<pre highlight="css">
		.container {
			position: relative;
			column-count: 3;
		}
		img {
			position: absolute;
			top: 20px;
			left: 40px;
		}
		</pre>

		<figure>
			<img src="images/column-not-containing-block.svg" alt="The absolutely positioned image is positioned by reference to the [=multi-column container=] not the [=column box=]." no-autosize>
			<figcaption>The figure demonstrates that the absolutely positioned image is positioned by reference to the multicol container and not the column box.</figcaption>
		</figure>
	</div>

	Out-of-flow descendants of a [=multi-column container=] do affect column balancing,
	and the block-size of the [=multi-column container=].

	<wpt>
	multicol-oof-inline-cb-001.html
	multicol-oof-inline-cb-002.html
	</wpt>

	The column boxes are ordered
	in the [=inline base direction=] of the multicol container
	and arranged into <dfn lt="multi-column line | multi-col line | multicol line">multicol lines</dfn>.
	The <dfn export>column width</dfn> is the length of the column box in the inline direction.
	The <dfn export>column height</dfn> is the length of the column box in the block direction.
	All column boxes in a line have the same column width,
	and all column boxes in a line have the same column height.

	<wpt title="The following tests relate to baseline alignment of the content of columns, 
	though this is not defined in this specification.">
	baseline-000.html
	baseline-001.html
	baseline-002.html
	baseline-003.html
	baseline-004.html
	baseline-005.html
	baseline-006.html
	baseline-007.html
	baseline-008.html
	crashtests/as-baseline-aligned-grid-item.html
	</wpt>

	<wpt title="The following tests check the behavior of list items that are also muticol containers.">
	multicol-list-item-001.xht
	multicol-list-item-002.html
	multicol-list-item-003.html
	multicol-list-item-004.html
	multicol-list-item-005.html
	multicol-list-item-006.html
	multicol-list-item-007.html
	multicol-list-item-008.html
	</wpt>

	<wpt title="Testing grid items inside multicol">
	balance-grid-001.html
	</wpt>

	<wpt title="The following tests check the behavior of table elements.">
	table/multicol-table-cell-001.xht
	table/multicol-table-cell-height-001.xht
	table/multicol-table-cell-height-002.xht
	table/multicol-table-cell-vertical-align-001.xht
	table/table-cell-as-multicol.html
	table/table-cell-content-change-000.html
	table/table-cell-content-change-001.html
	table/table-cell-multicol-nested-001.html
	table/table-cell-multicol-nested-002.html
	table/table-cell-multicol-nested-003.html
	crashtests/break-before-multicol-caption.html
	crashtests/table-cell-writing-mode-root.html
	table/balance-breakafter-before-table-section-crash.html
	table/balance-table-with-border-spacing.html
	table/balance-table-with-fractional-height-row.html
	</wpt>

	<wpt title="The following tests check that paint order is correct.">
	float-and-block.html
	move-with-text-after-paint.html
	resize-with-text-after-paint.html
	</wpt>

	<wpt title="The following tests relate to animation or transformation of multicol properties.">
	animation/column-width-interpolation.html
	animation/discrete-no-interpolation.html
	multicol-overflow-positioned-transform-001.html
	multicol-overflow-transform-001.html
	resize-multicol-with-fixed-size-children.html
	resize-in-strict-containment-nested.html
	remove-child-in-strict-containment-also-spanner.html
	composited-under-clip-under-multicol.html
	change-intrinsic-width.html
	change-fragmentainer-size-000.html
	change-fragmentainer-size-001.html
	change-fragmentainer-size-002.html
	change-fragmentainer-size-003.html
	hit-test-child-under-perspective.html
	hit-test-transformed-child.html
	</wpt>

	<wpt title="Tests related to implementation bugs, not linked to specific normative text.">
	crashtests/multicol-with-monolithic-oof-with-multicol-with-oof.html
	crashtests/chrome-bug-1293905.html
	crashtests/chrome-bug-1297118.html
	crashtests/chrome-bug-1301281.html
	crashtests/chrome-bug-1303256.html
	crashtests/chrome-bug-1314866.html
	crashtests/dynamic-simplified-layout-break-propagation.html
	crashtests/float-multicol-crash.html
	change-abspos-width-in-second-column-crash.html
	change-out-of-flow-type-and-remove-inner-multicol-crash.html
	crashtests/monolithic-oof-in-clipped-container.html
	crashtests/move-linebreak-to-different-column.html
	crashtests/move-newline-pre-text.html
	crashtests/multicol-at-page-boundary-print.html
	crashtests/multicol-block-in-inline-crash.html
	crashtests/multicol-cached-consumed-bsize-crash.html
	crashtests/multicol-column-change-crash.html
	crashtests/multicol-dynamic-contain-crash.html
	crashtests/multicol-dynamic-transform-crash.html
	crashtests/multicol-floats-in-ifc.html
	crashtests/multicol-parallel-flow-after-spanner-in-inline.html
	crashtests/outline-move-oof-with-inline.html
	crashtests/relpos-inline-with-abspos-multicol-gets-block-child.html
	crashtests/size-containment-become-multicol-add-inline-child.html
	crashtests/sticky-in-abs-in-sticky.html
	crashtests/vertical-rl-column-rules-wide-columns.html
	dynamic-become-multicol-add-oof-inside-inline-crash.html
	extremely-tall-multicol-with-extremely-tall-child-crash.html
	file-control-crash.html
	img-alt-as-multicol-crash.html
	overflow-scroll-in-multicol-crash.html
	remove-block-sibling-of-inline-with-block-crash.html
	subpixel-scroll-crash.html
	text-child-crash.html
	with-custom-layout-on-same-element-crash.https.html
	crashtests/oof-in-area-001.html
	crashtests/oof-in-area-002.html
	crashtests/oof-in-area-003.html
	crashtests/oof-in-oof-multicol-in-multicol-spanner-in-multicol.html
	crashtests/oof-in-oof-multicol-in-relpos-in-oof-in-multicol-in-multicol.html
	crashtests/oof-in-oof-multicol-in-relpos-spanner-in-multicol-in-relpos-multicol-in-multicol.html
	crashtests/oof-in-oof-multicol-in-relpos-spanner-in-spanner-multicol-in-multicol-in-multicol.html
	crashtests/oof-in-oof-multicol-in-spanner-in-multicol-in-spanner-in-nested-multicol.html
	crashtests/oof-in-oof-multicol-in-spanner-in-nested-multicol.html
	crashtests/oof-in-relpos-in-oof-multicol-in-oof-in-relpos-in-oof-multicol-in-multicol.html
	crashtests/oof-in-relpos-in-oof-multicol-in-relpos-in-oof-multicol-in-relpos-multicol.html
	crashtests/floated-input-in-inline-next-column.html
	crashtests/inline-float-parallel-flow.html
	crashtests/table-caption-in-clipped-overflow.html
	crashtests/text-in-inline-interrupted-by-float.html
	increase-prev-sibling-height.html
	crashtests/interleaved-bfc-crash.html
	crashtests/relayout-fixedpos-in-abspos-in-relpos-in-nested-multicol.html
	crashtests/inline-become-oof-container-make-oof-inflow.html
	multicol-dynamic-change-inside-break-inside-avoid-001.html
	multicol-dynamic-add-001.html
	scroll-width-height.tentative.html
	filter-with-abspos.html
	crashtests/add-list-item-marker.html
	</wpt>

	<wpt title="Tests related to printing and paged media as related to multicol.">
	auto-fill-auto-size-001-print.html
	auto-fill-auto-size-002-print.html
	multicol-height-002-print.xht
	named-page.html
	page-property-ignored.html
	</wpt>


	Note: In text set using a vertical writing mode,
	the block direction runs horizontally.
	In a vertical writing mode columns are laid out horizontally,
	and the direction of the flow of blocks may be right to left, or left to right.
	The 'column-width' property therefore refers to the inline size of the column,
	and not the physical horizontal width.

	<div class="example">
		<figure>
			<img alt="The first image shows horizontal text with a LTR inline direction. The second shows vertical text with blocks flowing right to left. The third shows vertical text with blocks flowing left to right." src="images/writing-modes.svg" no-autosize>
			<figcaption>A diagram showing the different ways columns may be arranged due to writing mode.
			<br>From left to right: horizontal-tb, vertical-rl, vertical-lr.</figcaption>
		</figure>
	</div>

	<wpt title="Tests regarding vertical writing modes.">
	orthogonal-writing-mode-shrink-to-fit.html
	multicol-under-vertical-rl-scroll.html
	hit-test-in-vertical-rl.html
	</wpt>

	Within each [=multicol line=] in the multi-column container,
	adjacent column boxes are separated by a <dfn noexport>column gap</dfn>,
	which may contain a <dfn noexport>column rule</dfn>.
	All column gaps in the same multi-column container are equal.
	All column rules in the same multi-column container are also equal, if they appear;
	column rules only appear between columns that both have content.

	In the simplest case a multicol container will contain only one line
	of columns, and the height of each column will be equivalent to the
	used height of the multi-column container's content box.
	However, [=fragmentation=] or [=spanners=]
	can split the content of the [=multi-column container=]
	into multiple [=multicol lines=].

	If the multi-column container is paginated, the height of each column is
	constrained by the page and the content continues in a new line of
	column boxes on the next page; a column box never splits across pages.

	The same effect occurs when a <i>spanning element</i> divides the
	multi-column container: the columns before the spanning element are
	balanced and shortened to fit their content. Content after the
	spanning element then flows into a new, subsequent line of column boxes.

	<div class="example">
		<figure>
			<img alt="a diagram showing a spanning element causing the shortened columns above the element with text continuing in new columns below" src="images/simple-span-example.svg" no-autosize>
			<figcaption>A demonstration of how the spanning element divides the multicol container.</figcaption>
		</figure>
	</div>

	A [=multi-column container=] therefore is a regular [=block container=]
	that establishes a new [=independent formatting context=]
	whose contents consist of a series of
	[=multicol lines=] and multicol spanners.
	Each [=multi-column line=] acts as a [=block-level box=]
	that establishes a <dfn>multi-column formatting context</dfn>
	for its [=column boxes=];
	and each [=spanner=] acts as a [=block-level box=]
	that establishes an [=independent formatting context=]
	with its type depending on its 'display' value as usual.

	Nested multi-column containers are allowed,
	but there may be implementation-specific limits.

	<wpt>
	multicol-nested-002.xht
	multicol-nested-005.xht
	multicol-nested-006.html
	multicol-nested-007.html
	multicol-nested-008.html
	multicol-nested-009.html
	multicol-nested-010.html
	multicol-nested-011.html
	multicol-nested-012.html
	multicol-nested-013.html
	multicol-nested-014.html
	multicol-nested-015.html
	multicol-nested-016.html
	multicol-nested-017.html
	multicol-nested-018.html
	multicol-nested-019.html
	multicol-nested-020.html
	multicol-nested-021.html
	multicol-nested-022.html
	multicol-nested-023.html
	multicol-nested-024.html
	multicol-nested-025.html
	multicol-nested-026.html
	multicol-nested-027.html
	multicol-nested-028.html
	multicol-nested-029.html
	multicol-nested-030.html
	multicol-nested-031.html
	crashtests/nested-as-balanced-legend.html
	crashtests/nested-as-nested-balanced-legend.html
	crashtests/nested-floated-multicol-with-tall-margin.html
	crashtests/nested-multicol-and-float-with-tall-padding-before-float.html
	crashtests/nested-multicol-and-float-with-tall-padding.html
	crashtests/nested-multicol-fieldset-tall-trailing-border-freeze.html
	crashtests/nested-multicol-fieldset-tall-trailing-padding.html
	crashtests/nested-multicol-in-svg-foreignobject.html
	crashtests/nested-multicol-nested-flex.html
	crashtests/nested-multicol-with-float-between.html
	crashtests/nested-oof-multicol-with-monolithic-child.html
	crashtests/nested-oof-multicol-with-oof-needing-additional-columns.html
	crashtests/nested-oof-multicol-with-padding.html
	crashtests/nested-with-fragmented-oof-negative-top-offset.html
	crashtests/nested-with-multicol-table-caption.html
	crashtests/multicol-table-caption-parallel-flow-after-spanner-in-inline.html
	crashtests/nested-with-multicol-table-cell.html
	crashtests/nested-with-oof-inside-fixed-width.html
	crashtests/nested-with-percentage-size-and-oof.html
	crashtests/nested-with-tall-padding-and-oof.html
	crashtests/nested-with-tall-padding.html
	crashtests/oof-in-nested-line-float.html
	crashtests/oof-nested-multicol-inside-oof.html
	crashtests/relayout-nested-with-oof.html
	crashtests/repeated-section-in-nested-table-nested-multicol.html
	crashtests/repeated-table-footer-in-caption-nested-multicol.html
	nested-balanced-monolithic-multicol-crash.html
	nested-balanced-very-tall-content-crash.html
	nested-floated-shape-outside-multicol-with-monolithic-child-crash.html
	nested-with-overflowing-padding-crash.html
	triply-nested-with-fixedpos-in-abspos-crash.html
	fixed-in-nested-multicol-with-transform-container.html
	fixed-in-nested-multicol-with-viewport-container.html
	fixed-in-nested-multicol.html
	nested-after-float-clearance.html
	nested-at-outer-boundary-as-fieldset.html
	nested-at-outer-boundary-as-float.html
	nested-at-outer-boundary-as-legend.html
	nested-floated-multicol-with-monolithic-child.html
	nested-oofs-in-relative-multicol.html
	nested-past-fragmentation-line.html
	nested-with-too-tall-line.html
	oof-nested-in-single-column.html
	nested-non-auto-inline-size-offset-top.html
	fixedpos-static-pos-with-viewport-cb-001.html
	fixedpos-static-pos-with-viewport-cb-002.html
	fixedpos-static-pos-with-viewport-cb-003.html
	multicol-height-block-child-001.xht
	</wpt>

	Note: It is not possible to set properties/values on column boxes.
	For example, the background of a certain column box cannot be set
	and a column box has no concept of padding, margin or borders.
	Future specifications may add additional functionality.
	For example, columns of different widths and different backgrounds may be supported.

	Note: Multicol containers with column heights larger than the viewport may pose accessibility issues.
	See <a href="#a11y">Accessibility Considerations</a> for more details.

<h2 id="the-number-and-width-of-columns">
The number and width of columns</h2>

	Finding the number and width of columns is fundamental when laying out multi-column content.
	These properties are used to set the number and width of columns:

	<ul>
		<li>'column-count'</li>
		<li>'column-width'</li>
	</ul>

	A third property, 'columns',
	is a shorthand property which sets both 'column-width' and 'column-count'.

	Other factors, such as explicit column breaks, content, and height constraints,
	may influence the actual number and width of columns.


<h3 id='cw'>The Inline Size of Columns: the 'column-width' property</h3>

	<pre class=propdef>
	Name: column-width
	Value: auto | <<length [0,&infin;]>>
	Initial: auto
	Applies to: <a>block containers</a> except <a>table wrapper boxes</a>
	Inherited: no
	Percentages: N/A
	Computed value: the keyword ''auto'' or an absolute length
	Animation type: by computed value type
	</pre>

	This property describes the width of columns in multicol containers.

	<dl dfn-type=value dfn-for=column-width>
		<dt><dfn>auto</dfn></dt>
		<dd>
			means that the column width will be determined by other properties
			(e.g., 'column-count', if it has a non-auto value).

		<dt><dfn><<length [0,&infin;]>></dfn>
		<dd>
			describes the optimal column width.
			The actual column width may be wider (to fill the available space),
			or narrower (only if the available space is smaller than the specified column width).
			Negative values are not allowed.
			Used values will be clamped to a minimum of ''1px''.

			<wpt>
			zero-column-width-computed-style.html
			zero-column-width-layout.html
			</wpt>
		</dd>
	</dl>

	<wpt>
	multicol-basic-003.html
	multicol-basic-008.xht
	multicol-reduce-000.xht
	multicol-width-001.xht
	multicol-width-002.xht
	multicol-width-003.xht
	multicol-width-005.html
	multicol-width-ch-001.xht
	multicol-width-negative-001.xht
	multicol-width-invalid-001.xht
	multicol-width-large-001.xht
	multicol-width-large-002.xht
	multicol-inherit-003.xht
	parsing/column-width-computed.html
	parsing/column-width-invalid.html
	parsing/column-width-valid.html
	multicol-width-small-001.xht
	</wpt>

	<div class="example">
		For example, consider this style sheet:

		<pre highlight="css">
		div {
			width: 100px;
			column-width: 45px;
			column-gap: 0;
			column-rule: none;
		}
		</pre>

		There is room for two ''45px'' wide columns inside the ''100px'' wide element.
		In order to fill the available space
		the actual column width will be increased to ''50px''.
	</div>

	<div class="example">
		Also, consider this style sheet:

		<pre highlight="css">
		div {
			width: 40px;
			column-width: 45px;
			column-gap: 0;
			column-rule: none;
		}
		</pre>

		The available space is smaller than the specified column width
		and the actual column width will therefore be decreased.
	</div>

	To ensure that 'column-width' can be used with vertical text,
	column width means the length of the line boxes inside the columns.

	Note: The reason for making 'column-width' somewhat flexible
	is to achieve scalable designs that can fit many screen sizes.
	To set an exact column width,
	the column gap and the width of the multicol container (assuming horizontal text)
	must also be specified.

<h3 id='ch'>The Block Size of Columns: the 'column-height' property</h3>

	<pre class=propdef>
	Name: column-height
	Value: auto | <<length [0,&infin;]>>
	Initial: auto
	Applies to: <a>block containers</a> except <a>table wrapper boxes</a>
	Inherited: no
	Percentages: N/A
	Computed value: the keyword ''auto'' or an absolute length
	Animation type: by computed value type
	</pre>

	This property describes the height of columns in multicol containers.

	<dl dfn-type=value dfn-for=column-height>
		<dt><dfn>auto</dfn></dt>
		<dd>
			means that the column height will be determined by the amount of content
			and other properties.

		<dt><dfn><<length [0,&infin;]>></dfn>
		<dd>
			describes the desired column height.
			Negative values are not allowed.
		</dd>
	</dl>

	If you set 'column-height' and 'height' on a multicol container,
	you may end up with some additional space.
	The properties defined in the [[CSS-ALIGN]] specification detail how to deal with it.

<h3 id='cc'>The Number of Columns: the 'column-count' property</h3>

	<pre class=propdef>
	Name: column-count
	Value: auto | <<integer [1,&infin;]>>
	Initial: auto
	Applies to: <a>block containers</a> except <a>table wrapper boxes</a>
	Inherited: no
	Percentages: N/A
	Computed value: specified value
	Animation Type: by computed value
	</pre>

	This property describes the number of columns of a [=multicol container=].

	<dl dfn-type=value dfn-for=column-count>
		<dt><dfn>auto</dfn>
		<dd>
			means that the number of columns will be determined by other properties
			(e.g., 'column-width', if it has a non-auto value).

		<dt><dfn><<integer [1,&infin;]>></dfn>
		<dd>
			describes the optimal number of columns into which the content of the element will be flowed.
			Values must be greater than 0.
			If both 'column-width' and 'column-count' have non-auto values,
			the integer value describes the maximum number of columns.
		</dd>
	</dl>

	<wpt>
	multicol-count-001.xht
	multicol-count-002.xht
	multicol-basic-006.xht
	multicol-width-count-001.xht
	multicol-width-count-002.xht
	multicol-columns-toolong-001.xht
	multicol-count-negative-001.xht
	multicol-count-negative-002.xht
	multicol-count-non-integer-001.xht
	multicol-count-non-integer-002.xht
	multicol-count-non-integer-003.xht
	multicol-inherit-001.xht
	multicol-inherit-002.xht
	parsing/column-count-invalid.html
	parsing/column-count-valid.html
	parsing/column-count-computed.html
	animation/column-count-interpolation.html
	large-actual-column-count.html
	with-custom-layout-on-same-element.https.html
	</wpt>

	<div class="example">
		Example:

		<pre highlight="css">body { column-count: 3 }</pre>
	</div>

<h3 id='cwr'>The overflow direction in continuous media: the 'column-wrap' property</h3>

	<pre class=propdef>
	Name: column-wrap
	Value: nowrap | wrap
	Initial: nowrap
	Applies to: multicol containers
	Inherited: no
	Percentages: n/a
	Computed value: specified keyword
	Canonical order: per grammar
	Animation type: discrete
	</pre>

	The column-wrap property controls the behavior of overflow columns in continuous media.
	The property only takes effect if 'column-height' is not 'auto',
	and there is more content than will fit in a single row of columns at the specified 'column-height'.

	<dl dfn-type=value dfn-for=column-wrap>
		<dt><dfn>nowrap</dfn>
		<dd>
			Overflow columns are created in the inline direction.

		<dt><dfn>wrap</dfn>
		<dd>
			Overflow columns create a new multicol row in the block direction.
		</dd>
	</dl>

	<div class="example">
	Example:

	<pre highlight="css">
	body {
		column-count: 3;
		column-wrap: nowrap;
		column-height: 10em;
	}
	</pre>

	This CSS creates a set of three columns.
	If there is more content than would fit in the 10em height of these columns,
	new columns are created in the inline direction
	even if there is room in the multicol container.
	
	</div>

	<div class="example">
	Example:

	<pre highlight="css">
	body {
  		column-count: 3;
  		column-wrap: wrap;
		column-height: 10em;
	}
	</pre>

	This CSS creates a set of three columns.
	If there is more content than would fit in the 10em height of these columns,
	rather than overflowing in the inline direction, 
	a new row of columns is created in the block direction, again at a max column-height of 10em.
	New rows repeat until all content is displayed.
	</div>

	Issue(11754): The addition of this property is as yet unresolved.
	
	Issue(11754): What happens if you set a height on the multicol container, 'column-wrap: wrap' but no 'column-height'?
	Options are 1: The 'column-wrap' property is ignored. 2: 'column-height: auto' resolves to the available space
	in the multicol container and new rows of that size are created for the overflow,
	thus honoring the 'wrap' value.

<h3 id="columns">The 'column-width', 'column-count', 'column-height', and 'column-wrap' Shorthand: The 'columns' Property</h3>

	<pre class="propdef shorthand">
	Name: columns
	Value: <<'column-width'>> || <<'column-count'>> || <<'column-height'>> || <<'column-wrap'>>
	</pre>

	This is a shorthand property for setting 'column-width', 'column-count',
	'column-height', and 'column-wrap'.
	Omitted values are set to their initial values.

	<div class="example">
		Here are some valid declarations using the 'columns' property:

		<pre highlight="css">
		columns: 12em;      /* column-width: 12em; column-count: auto */
		columns: auto 12em; /* column-width: 12em; column-count: auto */
		columns: 2;         /* column-width: auto; column-count: 2 */
		columns: 2 auto;    /* column-width: auto; column-count: 2 */
		columns: auto;      /* column-width: auto; column-count: auto */
		columns: auto auto; /* column-width: auto; column-count: auto */
		columns: 12em 14em; /* column-width: 12em; column-height: 14em; */
		columns: 12em 14em wrap;  /* column-width: 12em; column-height: 14em; column-wrap: wrap; */
		</pre>
	</div>

	<wpt>
	multicol-columns-001.xht
	multicol-columns-002.xht
	multicol-columns-003.xht
	multicol-columns-004.xht
	multicol-columns-005.xht
	multicol-columns-006.xht
	multicol-columns-007.xht
	multicol-columns-invalid-001.xht
	multicol-columns-invalid-002.xht
	multicol-basic-001.html
	multicol-basic-003.html
	multicol-basic-005.xht
	multicol-basic-007.xht
	parsing/columns-invalid.html
	parsing/columns-valid.html
	</wpt>

<h3 id="pseudo-algorithm">The Pseudo-algorithm</h3>

	The pseudo-algorithm below determines the used values for
	'column-count' (N) and 'column-width' (W). There is one other variable
	in the pseudo-algorithm: U is the used width of the multi-column container.

	Note: The used width U of the multi-column container can depend on the element's contents,
	in which case it also depends on the computed values of the 'column-count' and 'column-width' properties.
	This specification does not define how U is calculated.
	Another module (probably the Basic Box Model [[CSS3BOX]]
	or the Box Sizing Module [[CSS3-SIZING]]) is expected to define this.

<wpt>
intrinsic-size-001.html
intrinsic-size-002.html
intrinsic-size-003.html
intrinsic-size-004.html
intrinsic-size-005.html
as-column-flex-item.html
intrinsic-width-change-column-count.html
</wpt>

<!--
Two assumptions are being made by the pseudo-algorithm:

<ul>
<li>that the block direction is unconstrained
<li>that no column breaks are added through style sheets
</ul>
-->

	The <code>floor(X)</code> function returns the largest integer Y &le; X.

	<pre>
	(01)  if ((column-width = auto) and (column-count = auto)) then
	(02)      exit; /* not a multicol container */
	(03)  if column-width = auto then
	(04)      N := column-count
	(05)  else if column-count = auto then
	(06)      N := max(1,
	(07)        floor((U + column-gap)/(column-width + column-gap)))
	(08)  else
	(09)      N := min(column-count, max(1,
	(10)        floor((U + column-gap)/(column-width + column-gap))))
	</pre>

	And:

	<pre>
	(11)  W := max(0, ((U + column-gap)/N - column-gap))
	</pre>

	For the purpose of finding the number of auto-repeated columns,
	the UA must floor the column size to a UA-specified value to avoid division by zero.
	It is suggested that this floor be 1px or less.

	In fragmented contexts such as in [=paged media=],
	user agents may perform this calculation on a per-fragment basis.

	The used value for 'column-count' is calculated without regard for explicit column breaks or constrained column heights,
	while the actual value takes these into consideration.

	<wpt>
	column-count-used-001.html
	</wpt>

	<div class="example">
		In this example, the actual column-count is higher than the used column-count
		due to explicit column breaks:

		<pre highlight="css">
		div {
			width: 40em;
			columns: 20em;
			column-gap: 0;
		}

		p {
			break-after: column;
		}
		</pre>

		<pre highlight="html">
		&lt;div>
			&lt;p>one
			&lt;p>two
			&lt;p>three
		&lt;/div>
		</pre>
		<figure>
			<img src="images/column-count-higher-than-used-count.svg" alt="Two columns drawn inside the container, one outside" no-autosize>
			<figcaption>The computed column-count is auto, the used column-count is 2 and the actual column-count is 3.</figcaption>
		</figure>
	</div>

	<div class=example>
		The actual column-count may be lower than the used column-count.
		Consider this example:

		<pre highlight="css">
		div {
			width: 80em;
			height: 10em;
			columns: 20em;
			column-gap: 0;
			column-fill: auto;
		}
		</pre>
		<pre highlight="html">
		&lt;div>foo&lt;/div>
		</pre>

		The computed column-count is auto,
		the used column-count is 4,
		and the actual column-count is 1.
	</div>

<h3 id="stacking-context">
Stacking Context</h3>

	All column boxes in a multi-column container are in the same stacking context
	and the drawing order of their contents is as specified in CSS 2.1.
	Column boxes do not establish new stacking contexts.

	<wpt>
	multicol-rule-stacking-001.xht
	</wpt>

<h3 id=column-pseudo>
Styling Columns: the ''::column'' pseudo-element</h3>

The <dfn selector>::column</dfn> pseudo-element
represents the individual columns
in a [=multi-column container=].
It only exists on [=multi-column containers=].

A [=multi-column container=] has as many ''::column'' pseudo-elements
as it has columns.
They cannot be individually addressed;
any styles applied to a multicol's ''::column'' pseudo-elements
apply to all of them on that element.
Each ''::column'' has the same size and position
as its corresponding column:
the same size as the column's [=available space=] in the [=inline axis=]
and the container's [=content box=] in the [=block axis=].
(It does not cover the gaps/rules between columns.)

''::column'' pseudo-elements are treated as children of their [=multi-column container=],
without any contents.
They <em>do not</em> wrap the contents of the column,
just fill the same space.

The ''::column'' pseudo-element accepts only a very limited set of properties:

* 'scroll-margin', 'scroll-snap-align', and 'scroll-snap-stop' 
	(the scroll snap properties that apply to elements <em>inside</em> a [=scroll container=])

Additionally, a ''::column'' pseudo-element can have a ''::scroll-marker'' pseudo-element of its own,
as ''::column::scroll-marker''.
(Other pseudo-elements do not exist on ''::column''.)
Such ''::scroll-marker'' pseudo-elements
inherit from the ''::column'' pseudo-element's [=originating element=],
rather than the ''::column'' itself.

Note: This list of properties and pseudo-elements that work on ''::column''
will likely be expanded in the future,
as we develop more features
that could usefully care about the <em>position</em> of a column
without caring about the <em>contents</em>.

<h2 id="column-gaps-and-rules">Column Gaps and Rules</h2>

	Column gaps and rules are placed between columns in the same [=multicol container=].
	The length of the column gaps and column rules is equal to the column height.
	Column gaps take up space.
	That is, column gaps will push apart content in adjacent columns
	(within the same [=multicol container=]).

	<wpt>
	multicol-height-001.xht
	multicol-nested-column-rule-001.xht
	multicol-nested-column-rule-002.html
	multicol-nested-column-rule-003.html
	multicol-rule-nested-balancing-001.html
	multicol-rule-nested-balancing-002.html
	multicol-rule-nested-balancing-003.html
	multicol-rule-nested-balancing-004.html
	</wpt>

	A [=column rule=] is drawn in the middle of the [=column gap=]
	with the endpoints at opposing content edges of the [=multicol container=].
	Column rules do not take up space.
	That is, the presence or thickness of a [=column rule=] will not alter the placement of anything else.
	If a [=column rule=] is wider than its gap,
	the adjacent column boxes will overlap the rule,
	and the rule may possibly extend outside the box of the [=multicol container=].
	Column rules are painted just above the border of the [=multicol container=].
	For scrollable multicol containers,
	note that while the border and background of the [=multicol container=] obviously aren't scrolled,
	the rules need to scroll along with the columns.
	Column rules are only drawn between two columns that both have content.

	<wpt title="Basic column rule tests">
	multicol-rule-003.xht
	multicol-rule-004.xht
	multicol-rule-fraction-002.xht
	</wpt>

	<wpt title="If a column rule is wider than its gap, adjacent boxes overlap.">
	multicol-rule-001.xht
	multicol-rule-large-001.xht
	multicol-rule-large-002.xht
	</wpt>

	<wpt title="Column rules are only drawn between two columns that have content.">
	multicol-count-computed-003.xht
	multicol-count-computed-005.xht
	broken-column-rule-1.html
	</wpt>

	<wpt title="Tests for behavior of backgrounds and column rules.">
	multicol-breaking-000.html
	multicol-breaking-001.html
	multicol-breaking-002.html
	multicol-breaking-003.html
	multicol-breaking-004.html
	multicol-breaking-005.html
	multicol-breaking-006.html
	multicol-breaking-nobackground-000.html
	multicol-breaking-nobackground-001.html
	multicol-breaking-nobackground-002.html
	multicol-breaking-nobackground-003.html
	multicol-breaking-nobackground-004.html
	multicol-breaking-nobackground-005.html
	</wpt>

<h3 id='cg'>Gutters Between Columns: the 'column-gap' property</h3>

	The 'column-gap' property is defined in [[!CSS3-ALIGN]].

	In a [=multi-column formatting context=]
	the used value of ''column-gap/normal'' for the 'column-gap' property is ''1em''.
	This ensures columns are readable when the initial values are used.
	If there is a column rule between columns,
	it will appear in the middle of the gap.

	<wpt>
	multicol-gap-fraction-001.xht
	multicol-gap-fraction-002.html
	multicol-gap-large-001.xht
	multicol-gap-large-002.xht
	multicol-gap-negative-001.xht
	multicol-gap-000.xht
	multicol-gap-002.xht
	multicol-gap-percentage-001.html
	multicol-gap-001.xht
	multicol-gap-003.xht
	</wpt>

	<wpt title="Tests that the gap is animatable.">
	multicol-gap-animation-001.html
	multicol-gap-animation-002.html
	multicol-gap-animation-003.html
	</wpt>

<h3 id='cg'>Gutters Between Rows: the 'row-gap' property</h3>

	The 'row-gap' property is defined in [[!CSS3-ALIGN]].

	In a [=multi-column formatting context=]
	the used value of ''row-gap/normal'' for the 'row-gap' property is ''1em''.
	If there is a rule between rows,
	it will appear in the middle of the gap.

<h3 id='crc'>The Color of Column Rules: the
'column-rule-color' property</h3>

	<pre class=propdef>
	Name: column-rule-color
	Value: <<color>>
	Initial: currentcolor
	Applies to: multicol containers
	Inherited: no
	Percentages: N/A
	Computed value: computed color
	Animation type: by computed value type
	</pre>

	<dl>
		<dt><dfn value for=column-rule-color><<color>></dfn></dt>
		<dd>
			Specifies the color of the [=column rule=].
		</dd>
	</dl>

	<wpt>
	multicol-rule-color-001.xht
	multicol-rule-color-inherit-001.xht
	multicol-rule-color-inherit-002.xht
	parsing/column-rule-color-computed.html
	parsing/column-rule-color-valid.html
	parsing/column-rule-color-invalid.html
	animation/column-rule-color-interpolation.html
	</wpt>

<h3 id='crs'>The Style Of Column Rules: the 'column-rule-style' property</h3>

	<pre class=propdef>
	Name: column-rule-style
	Value: <<line-style>>
	Initial: none
	Applies to: multicol containers
	Inherited: no
	Percentages: N/A
	Computed value: specified keyword
	Animation type: discrete
	</pre>

	The 'column-rule-style' property sets the style of the rule between columns of an element.
	The <<line-style>> values are interpreted as in the <a href="https://www.w3.org/TR/CSS2/tables.html#collapsing-borders">collapsing border model</a>.

	<wpt>
	parsing/column-rule-style-computed.html
	parsing/column-rule-style-valid.html
	parsing/column-rule-style-invalid.html
	</wpt>

	The ''border-style/none'' and ''hidden'' values force the computed value of 'column-rule-width' to be ''0''.

<h3 id='crw'>The Width Of Column Rules: the 'column-rule-width' property</h3>

	<pre class=propdef>
	Name: column-rule-width
	Value: <<line-width>>
	Initial: medium
	Applies to: multicol containers
	Inherited: no
	Percentages: N/A
	Computed value: absolute length, [=snapped as a border width=]; ''0'' if the column rule style is ''border-style/none'' or ''hidden''
	Animation type: by computed value type
	</pre>

	This property sets the width of the rule between columns.
	Negative values are not allowed.

	<wpt>
	multicol-rule-fraction-001.xht
	multicol-rule-fraction-003.xht
	multicol-rule-px-001.xht
	multicol-rule-percent-001.xht
	subpixel-column-rule-width.tentative.html
	parsing/column-rule-width-computed.html
	parsing/column-rule-width-invalid.html
	parsing/column-rule-width-valid.html
	animation/column-rule-width-interpolation.html
	</wpt>

<h3 id="cr">Column Rule Shorthand: the 'column-rule' property</h3>

	<pre class="propdef shorthand">
	Name: column-rule
	Value: <<'column-rule-width'>> || <<'column-rule-style'>> || <<'column-rule-color'>>
	</pre>

	This property is a shorthand for setting
	'column-rule-width', 'column-rule-style', and 'column-rule-color'
	at the same place in the style sheet.
	Omitted values are set to their initial values.

	<wpt>
	
	multicol-shorthand-001.xht
	multicol-rule-shorthand-001.xht
	multicol-rule-shorthand-2.xht
	multicol-rule-000.xht
	multicol-rule-002.xht
	multicol-rule-dashed-000.xht
	multicol-rule-dotted-000.xht
	multicol-rule-double-000.xht
	multicol-rule-outset-000.xht
	multicol-rule-none-000.xht
	multicol-rule-hidden-000.xht
	multicol-rule-inset-000.xht
	multicol-rule-groove-000.xht
	multicol-rule-ridge-000.xht
	multicol-rule-solid-000.xht
	parsing/column-rule-computed.html
	parsing/column-rule-invalid.html
	parsing/column-rule-valid.html
	parsing/column-rule-shorthand.html
	</wpt>

	<div class=example>
		In this example, the [=column rule=] and the [=column gap=] have the same width.
		Therefore, they will occupy exactly the same space.

		<pre highlight="css">
		body {
			column-gap: 35px;
			column-rule-width: 35px;
			column-rule-style: solid;
			column-rule-color: black;
		}
		</pre>

		<figure>
			<img src="images/rule-same-width-as-gap.svg" alt="The rule completely covers any gap." no-autosize>
			<figcaption>The column rule and column gap occupy the same space.</figcaption>
		</figure>

		<wpt>
		equal-gap-and-rule.html
		multicol-rule-samelength-001.xht
		fixed-size-child-with-overflow.html
		</wpt>
	</div>

<h2 id="column-breaks">
Column Breaks</h2>

	When content is laid out in multiple columns,
	the user agent must determine where column breaks are placed.
	The problem of breaking content into columns is similar to breaking content into pages,
	which is described in CSS 2.1, section 13.3.3 [[!CSS21]].

	Three new properties are introduced to allow column breaks to be described in the same properties as page breaks:
	'break-before', 'break-after', and 'break-inside'.

<h3 id="break-before-break-after-break-inside">Controlling Fragmentation: the 'break-before', 'break-after', 'break-inside' properties</h3>

	'break-before', 'break-after', and 'break-inside'
	are defined in [[!CSS3-BREAK]].

	<wpt>
	multicol-break-000.xht
	multicol-break-001.xht
	multicol-br-inside-avoidcolumn-001.xht
	moz-multicol3-column-balancing-break-inside-avoid-1.html
	</wpt>

<h2 id="spanning-columns">
Spanning columns</h2>

	The 'column-span' property makes it possible for an element to span across several columns.
	This specification adds <<integer>> to the values available in the previous level.


<h3 id="column-span">
'column-span'</h3>

	<pre class="propdef">
	Name: column-span
	Value: none | <<integer [1,&infin;]>> | all | auto
	Initial: none
	Applies to: in-flow block-level elements
	Inherited: no
	Percentages: N/A
	Computed value: specified value
	Animation type: discrete
	</pre>

	This property describes how many columns an element spans across. Values are:

	<dl dfn-type=value dfn-for=column-span>
		<dt><dfn>none</dfn></dt>
		<dd>
			The element does not span multiple columns.

			<wpt>
			multicol-span-none-001.xht
			</wpt>
		</dd>
		<dt><dfn>all</dfn></dt>
		<dd>
			The element forces a column break and is taken [=out of flow=]
			to span across all columns of the nearest multicol ancestor
			in the same [=block formatting context=].
			Content in the normal flow that appears before the element
			is automatically balanced across all columns
			in the immediately preceding [=multi-column line=] before the element appears,
			and any subsequent content flows into a new [=multi-column line=] after the element.
			The element establishes an [=independent formatting context=].

			Note: Whether the element establishes a new <a>formatting context</a>
			does not depend on whether the element is a descendant of a multicol or not.
			When 'column-span' is ''column-span/all'', it always does.
			This helps with robustness of designs to later revisions that remove the multicol,
			or when media queries turn the multicol off in some situations.

			<wpt>
			multicol-span-000.xht
			multicol-span-all-001.xht
			multicol-span-all-003.xht
			multicol-span-all-block-sibling-003.xht
			multicol-span-all-margin-001.xht
			multicol-span-all-margin-002.xht
			multicol-span-all-margin-003.html
			multicol-span-all-margin-bottom-001.xht
			multicol-span-all-margin-nested-001.xht
			multicol-span-all-margin-nested-002.xht
			multicol-span-all-margin-nested-firstchild-001.xht
			multicol-span-float-001.xht
			multicol-span-float-002.html
			multicol-span-float-003.html
			inline-block-and-column-span-all.html
			multicol-span-all-dynamic-remove-001.html
			multicol-span-all-dynamic-add-001.html
			multicol-span-all-dynamic-remove-002.html
			multicol-span-all-dynamic-add-002.html
			multicol-span-all-dynamic-remove-003.html
			multicol-span-all-dynamic-add-003.html
			multicol-span-all-dynamic-remove-004.html
			multicol-span-all-dynamic-add-004.html
			multicol-span-all-dynamic-add-005.html
			multicol-span-all-dynamic-remove-005.html
			multicol-span-all-dynamic-remove-006.html
			multicol-span-all-dynamic-add-006.html
			multicol-span-all-dynamic-remove-007.html
			multicol-span-all-dynamic-add-007.html
			multicol-span-all-dynamic-add-008.html
			multicol-span-all-dynamic-add-009.html
			multicol-span-all-dynamic-add-010.html
			multicol-span-all-dynamic-add-011.html
			multicol-span-all-dynamic-add-012.html
			multicol-span-all-dynamic-add-013.html
			multicol-span-all-children-height-001.html
			multicol-span-all-children-height-002.html
			multicol-span-all-children-height-003.html
			multicol-span-all-children-height-004a.html
			multicol-span-all-children-height-004b.html
			multicol-span-all-children-height-005.html
			multicol-span-all-children-height-006.html
			multicol-span-all-children-height-007.html
			multicol-span-all-children-height-008.html
			multicol-span-all-children-height-009.html
			multicol-span-all-children-height-010.html
			multicol-span-all-children-height-011.html
			multicol-span-all-children-height-012.html
			multicol-span-all-children-height-013.html
			multicol-span-all-004.html
			multicol-span-all-005.html
			multicol-span-all-006.html
			multicol-span-all-007.html
			multicol-span-all-008.html
			multicol-span-all-009.html
			multicol-span-all-010.html
			multicol-span-all-011.html
			multicol-span-all-012.html
			multicol-span-all-013.html
			multicol-span-all-014.html
			multicol-span-all-015.html
			multicol-span-all-016.html
			multicol-span-all-017.html
			multicol-span-all-018.html
			multicol-span-all-019.html
			multicol-span-all-rule-001.html
			multicol-span-all-button-001.html
			multicol-span-all-button-002.html
			multicol-span-all-button-003.html
			multicol-span-all-fieldset-001.html
			multicol-span-all-fieldset-002.html
			multicol-span-all-fieldset-003.html
			multicol-span-all-restyle-001.html
			multicol-span-all-restyle-002.html
			multicol-span-all-restyle-003.html
			multicol-span-all-restyle-004.html
			multicol-span-all-list-item-001.html
			multicol-span-all-list-item-002.html
			float-with-line-after-spanner.html
			parallel-flow-after-spanner-001.html
			parallel-flow-after-spanner-002.html
			crashtests/margin-and-break-before-child-spanner.html
			multicol-width-004.html
			</wpt>
		</dd>



		<dt><dfn><<integer [1,&infin;]>></dfn>
		<dd>
			The element spans the specified number of columns.
			Values must be greater than 0.
			If the specified integer value is equal to,
			or larger than the number of columns in the multicol element,
			the number of columns spanned will be the same as if ''column-span: all'' had been specified.

			<div class=issue>This definition is insufficient.
			<ul>
				<li>Does ''column-span: 1'' count as ''column-span: none'',
				or does it create a spanner (which is a BFC)?
				<li>Which columns does it span?
				<li>How does that affect height calculations, and interact with column-fill
			</ul>
			</div>

		<dt><dfn>auto</dfn>
		<dd>
			The number of columns spanned by the element
			depends on its <a>min-content</a> <a>outer size</a>
			in the inline direction of the <a>multi-column container</a>.
			If it is smaller than the used value of 'column-width',
			this is the same as if ''column-span: none'' had been specified.
			Otherwise, the number of columns spanned is the smallest positive integer <var>n</var>
			for which <code><var>n</var> × <var ignore>'column-width'</var> + (<var>n</var> - 1) × <var ignore>'column-gap'</var></code>
			is larger than the <a>min-content</a> <a>outer size</a>.
			If this would be larger than the number of columns,
			the number of columns spanned will be the same as if ''column-span: all'' had been specified.

			Issue: If ''column-span: 1'' does not do the same as ''column-span: none'',
			should this behave as ''column-span: 1'' or as ''column-span: none''
			when the element is small enough?
	</dl>

	An element that spans more than one column is called a <dfn export local-lt="spanning element">multi-column spanning element</dfn>
	and the box it creates is called a <dfn export local-lt="spanner">multi-column spanner</dfn>.

	The [=containing block=] of the [=spanner=] is the [=multicol container=] itself.
	Consequently, in cases where the spanner itself does not establish
	a [=containing block=] for [=absolutely positioned boxes=] inside the spanner,
	their [=containing block chain=] skips directly to the [=multicol container=]
	(skipping any ancestors between the [=spanner=] and the [=multicol container=]).

	Although the spanner is taken [=out-of-flow=],
	this does not affect the <a href="https://www.w3.org/TR/CSS2/zindex.html">painting order</a> [[!CSS21]] of the spanning element.

	<div class="example">
		In this example, an <code>h2</code> element has been added to the sample document after the sixth sentence
		(i.e., after the words "the leg of a").
		This styling applies:

		<pre highlight="css">
		h2 { column-span: all; background: silver }
		</pre>

		By setting 'column-span' to ''column-span/all'',
		all content that appears before the <code>h2</code> element
		is shown above the <code>h2</code> element.

		<figure>
			<img src="images/h2-spanner.svg" alt="An element spans all three columns" no-autosize>
			<figcaption>The h2 element is set to column-span: all</figcaption>
		</figure>

		Note that because the spanner splits the [=multi-column line=],
		it also interrupts any [=column rules=]
		(which are only drawn between [=columns=] in a [=multi-column line=]).
	</div>

	A spanning element may be lower than the first level of descendants
	as long as they are part of the same [=formatting context=],
	and there is nothing between the spanning element and [=multicol container=] that establishes a containing block for fixed position descendants.

	<div class="example">
		In this example, the element with ''column-span: all'' is inside an element with ''transform: rotate(90deg)''. 
		The transform establishes a containing block for fixed position descendents, 
		therefore a spanner will not be created.

		<pre highlight="markup">
		&lt;article&gt;
			&lt;section&gt;
				&lt;div class="spanner"&gt;Attempted spanner&lt;/div&gt;
			&lt;/section&gt;
		&lt;/article&gt;
		</pre>

			<pre highlight="css">
article {
	columns: 2;
}

section {
	transform: rotate(90deg);
}

.spanner {
	column-span: all;
	background: silver;
}
</pre>
	</div>

	<wpt>
	fixed-in-multicol-with-transform-container.html
	change-transform-in-nested.html
	change-transform-in-second-column.html
	change-transform.html
	</wpt>

	If the fragment before the spanner is empty, nothing special happens;
	the top margin/border/padding is above the spanning element, as an empty [=fragment=].

	<div class="example">
		In this example the [=multicol container=] is the <code>article</code> element.
		Inside this parent is a paragraph and then a section element.
		The section contains an <code>h2</code> heading set to ''column-span/all''
		this spans all three columns while the containing section remains inside the column boxes.

		The <code>h2</code> is the first child of the section.
		This means that the margin,
		border (shown in red in the diagram)
		and padding on this section appear before the spanning <code>h2</code> as an empty fragment.

		<pre highlight="markup">
		&lt;article&gt;
			&lt;p&gt;...&lt;/p&gt;
			&lt;section&gt;
				&lt;h2&gt;An h2 element&lt;/h2&gt;
				&lt;p&gt;...&lt;/p&gt;
			&lt;/section&gt;
		&lt;/article&gt;
		</pre>

<pre highlight="css">
section {
	border: 2px solid red;
	margin-top: 65px;
	padding-top: 20px;
}

h2 {
	column-span: all;
	background: silver
}
</pre>

	<figure>
		<img src="images/nested-spanner.svg" alt="An element spans all three columns, the red border around the section breaks before the spanner." no-autosize>
		<figcaption>The <code>h2</code> element is set to ''column-span: all'',
		the section has a red border and top padding and margin</figcaption>
	</figure>

	</div>

	<wpt>
	spanner-fragmentation-000.html
	spanner-fragmentation-001.html
	spanner-fragmentation-002.html
	spanner-fragmentation-003.html
	spanner-fragmentation-004.html
	spanner-fragmentation-005.html
	spanner-fragmentation-006.html
	spanner-fragmentation-007.html
	spanner-fragmentation-008.html
	spanner-fragmentation-009.html
	spanner-fragmentation-010.html
	spanner-fragmentation-011.html
	spanner-fragmentation-012.html
	</wpt>

	A spanning element takes up more space than the element would take up otherwise.
	When space is limited, it may be impossible to find room for the spanning element.
	In these cases, user agents may treat the element as if ''column-span/none'' had been specified on this property.

	<div class="example">
		In this example, the <code>h2</code> element appears later in the content,
		and the height of the multicol container is constrained.
		Therefore, the <code>h2</code> element appears in the overflow
		and there is not room to make the element spanning.
		As a result, the element appears as if ''column-span: none'' was specified.

		<figure>
			<img src="images/h2-in-the-overflow-no-span.svg" alt="The h2 element is in an overflow column" style="max-height: 107px;" no-autosize>
			<figcaption>The h2 element is in an overflow column and appears as if column-span none is specified</figcaption>
		</figure>
	</div>

	<div class="example">
		This example is similar to the previous example,
		except that the H2 element appears naturally in the last column.
		Still, there is not enough room to make the element spanning.

		<figure>
			<img src="images/h2-in-the-last-column-no-span.svg" alt="The h2 element is in the final column" style="max-height: 107px;" no-autosize>
			<figcaption>The h2 element is in the final column and appears as if column-span none is specified</figcaption>
		</figure>
	</div>

	<wpt>
	multicol-span-all-002.xht
	</wpt>

	<div class="example">
		In fragmented contexts spanning elements are honored in all fragments.
		In this example, we are in [=paged media=],
		and the first three paragraphs have column breaks after them.
		A spanning <code>H2</code> element appears after the fourth paragraph.


		<figure>
			<img src="images/spanner-page-break1.svg" alt="Three columns with two lines of text each" no-autosize>
			<figcaption>This would appear on the first page</figcaption>
		</figure>

		<figure>
			<img src="images/spanner-page-break2.svg" alt="A spanning element across the three columns, text above and below." no-autosize>
			<figcaption>This would appear on the second page</figcaption>
		</figure>
	</div>

	Spanners are block-level boxes
	therefore the margins of two adjacent spanners will collapse with each other.
	The margins of two spanners separated only by an absolutely positioned item will collapse with each other,
	as absolutely positioned items do not create column boxes.
	As column boxes establish a new [=block formatting context=],
	margins on elements inside a column box will not collapse with the margin of a spanner.

	<wpt>
	non-adjacent-spanners-000.html
	non-adjacent-spanners-001.html
	</wpt>

	<div class="example">
		Spanners establish new [=formatting contexts=], but their margins can be changed by their surroundings.
		In this example, two spanners naturally end up at the top of a page.
		The top margin of the first spanner is truncated due to adjoining an unforced break.
		The margins between the two spanners collapse with each other.
		However, the bottom margin of the second spanner does not collapse with the top margin of the subsequent element.

		<pre highlight="css">
		h2 {
			margin: 16px 0;
			column-span: all;
			background: silver
		}
		p { margin-top: 16px }
		</pre>

		<figure>
			<img src="images/two-spanners-margin-no-collapse.svg" alt="Two spanning elements after a page break" no-autosize>
			<figcaption>Margins collapse between two spanning elements, but not the bottom margin of a spanner and top margin of next element.</figcaption>
		</figure>
	</div>

	<wpt title="Additional tests relating to spanners.">
	abspos-in-multicol-with-spanner-crash.html
	crashtests/body-becomes-spanner-html-becomes-vertical-rl.html
	crashtests/fit-content-with-spanner-and-auto-scrollbar-sibling.html
	crashtests/float-becomes-non-float-spanner-surprises-inside.html
	crashtests/float-becomes-spanner.html
	crashtests/multicol-floats-after-column-span-crash.html
	crashtests/negative-margin-on-column-spanner.html
	crashtests/nested-spanner-with-negative-margin.html
	crashtests/oof-becomes-spanner.html
	crashtests/oof-in-additional-column-before-spanner.html
	crashtests/relpos-spanner-with-spanner-child-becomes-regular.html
	crashtests/remove-spanner-after-spanner-in-inline-before-inline.html
	crashtests/remove-spanner-in-table-caption-nested-multicol.html
	crashtests/restricted-height-bottom-border-overflow-and-spanner.html
	crashtests/scrollable-spanner-in-nested.html
	crashtests/spanner-after-parallel-flow.html
	crashtests/spanner-in-inline-after-very-tall-content-001.html
	crashtests/spanner-in-inline-after-very-tall-content-002.html
	crashtests/spanner-in-overflowed-container-before-float.html
	crashtests/nested-with-tall-padding-and-spanner-and-content.html
	crashtests/specified-height-with-just-spanner-and-oof.html
	crashtests/trailing-parent-padding-between-spanners.html
	crashtests/table-caption-change-descendant-display-type.html
	crashtests/table-caption-inline-block-remove-child.html
	remove-block-beside-spanner-in-inline-crash.html
	remove-inline-with-block-beside-spanners-crash.html
	remove-spanner-beside-spanner-in-inline-crash.html
	spanning-legend-000-crash.html
	spanning-legend-001-crash.html
	toggle-spanner-float-crash.html
	abspos-after-spanner-static-pos.html
	abspos-after-spanner.html
	abspos-containing-block-outside-spanner.html
	change-transform-in-spanner.html
	nested-with-padding-and-spanner.html
	orthogonal-writing-mode-spanner.html
	remove-inline-with-block-beside-spanners.html
	replaced-content-spanner-auto-width.html
	spanner-in-child-after-parallel-flow-001.html
	spanner-in-child-after-parallel-flow-002.html
	spanner-in-child-after-parallel-flow-003.html
	spanner-in-child-after-parallel-flow-004.html
	spanner-in-opacity.html
	going-out-of-flow-after-spanner.html
	crashtests/inline-with-spanner-in-overflowed-container-before-multicol-float.html
	crashtests/spanner-in-overflowed-clipped-container.html
	crashtests/spanner-in-overflowed-container-before-inline-content.html
	crashtests/spanner-inside-inline-in-overflowed-container.html
	parsing/column-span-valid.html
	parsing/column-span-computed.html
	getclientrects-000.html
	getclientrects-001.html
	</wpt>

<h2 id="filling-columns">
Filling Columns</h2>

	There are two strategies for filling columns:
	columns can either be balanced, or not.
	If columns are balanced, user agents should try to minimize variations in column height,
	while honoring forced breaks,
	'widows' and 'orphans',
	and other properties that may affect column heights.
	If columns are not balanced, they are filled sequentially;
	some columns may end up partially filled, or with no content at all.

<h3 id='cf'>Column Balancing: the 'column-fill' property</h3>

	<pre class="propdef">
	Name: column-fill
	Value: auto | balance | balance-all
	Initial: balance
	Applies to: multicol containers
	Inherited: no
	Percentages: N/A
	Computed value: specified keyword
	Animation type: discrete
	</pre>

	This property specifies whether content
	in a [=multi-column line=] that does <em>not</em> immediately precede a [=spanner=]
	is balanced across columns or not.

<wpt>
always-balancing-before-column-span.html
no-balancing-after-column-span.html
</wpt>

	The values are:

	<dl dfn-type=value dfn-for=column-fill>
		<dt><dfn>balance</dfn></dt>
		<dd>
			Balance content equally between columns, as far as possible.
			In fragmented contexts, only the last fragment is balanced.

			<wpt>
			multicol-fill-000.xht
			multicol-fill-001.xht
			parsing/column-fill-invalid.html
			parsing/column-fill-valid.html
			parsing/column-fill-computed.html
			column-fill-balance-orthog-block-001.html
			column-balancing-paged-001-print.html
			multicol-fill-balance-001.xht
			multicol-fill-balance-002.html
			multicol-fill-balance-003.html
			multicol-fill-balance-004.html
			multicol-fill-balance-005.html
			multicol-fill-balance-006.html
			multicol-fill-balance-007.html
			multicol-fill-balance-008.html
			multicol-fill-balance-009.html
			multicol-fill-balance-010.html
			multicol-fill-balance-011.html
			multicol-fill-balance-012.html
			multicol-fill-balance-013.html
			multicol-fill-balance-014.html
			multicol-fill-balance-015.html
			multicol-fill-balance-016.html
			multicol-fill-balance-018.html
			multicol-fill-balance-019.html
			multicol-fill-balance-020.html
			multicol-fill-balance-021.html
			multicol-fill-balance-022.html
			multicol-fill-balance-023.html
			multicol-fill-balance-024.html
			multicol-fill-balance-025.html
			multicol-fill-balance-026.html
			multicol-fill-balance-027.html
			multicol-fill-balance-028.html
			multicol-fill-balance-nested-000.html
			</wpt>
		</dd>
		<dt><dfn>balance-all</dfn></dt>
		<dd>
			Balance content equally between columns, as far as possible.
			In fragmented contexts, all fragments are balanced.
		</dd>
		<dt><dfn>auto</dfn></dt>
		<dd>
			fill columns sequentially

			<wpt>
			multicol-fill-auto-001.xht
			multicol-fill-auto-002.xht
			multicol-fill-auto-003.xht
			multicol-fill-auto-004.html
			multicol-fill-auto-block-children-001.xht
			multicol-fill-auto-block-children-002.xht
			multicol-fill-auto-block-children-003.html
			columnfill-auto-max-height-001.html
			columnfill-auto-max-height-002.html
			columnfill-auto-max-height-003.html
			</wpt>
		</dd>
	</dl>

	In continuous contexts, this property does not have any effect when there are overflow columns.

	<div class="example">
		In this example, an article only has one short paragraph which fits on three lines.
		The three lines are displayed in three different
		columns due to column balancing.

		<pre highlight="css">
		article {
			width: 60em;
			height: auto;
			columns: 4;
			column-fill: balance;
		}
		</pre>

		<figure>
			<img src="images/column-balancing-one-paragraph.svg" alt="Four columns, the first three have content." no-autosize>
			<figcaption>Three lines displayed in three columns due to column balancing.</figcaption>
		</figure>

	</div>

	<div class="example">
		In this example, column balancing is turned off, and the article has a height:

		<pre highlight="css">
		article {
			width: 60em;
			height: 4em;
			columns: 4;
			column-fill: auto;
		}
		</pre>

		As a result, the first column is filled with all content:

		<figure>
			<img src="images/no-column-balancing-one-paragraph.svg" alt="Four columns, the first one has content." no-autosize>
			<figcaption>No balancing so the whole text is shown in one paragraph.</figcaption>
		</figure>
	
	</div>


	<div class=example>
		In this example, an article has two paragraphs:
		first a long one,
		then a shorter one.
		This code is applied:

		<pre highlight="css">
		article {
			width: 60em;
			height: auto;
			columns: 4;
			column-fill: balance;
		}

		p {
			break-after: column;
		}
		</pre>

		The shortest column height possible contains five lines of text.
		After the column height has been established, columns are filled sequentially.
		As a result, the third column is as high as the first two columns,
		while the last column ends up being significantly shorter.

		<figure>
			<img src="images/column-balancing-with-column-break.svg" alt="Four columns, all have content." no-autosize>
			<figcaption>Once column height is established, columns are filled sequentially.</figcaption>
		</figure>
	</div>

	<div class="example">
		<pre highlight="css">
		article {
			width: 60em;
			height: auto;
			columns: 4;
			column-fill: balance;
		}
		</pre>

		In this example, an article starts with an unbreakable figure which sets the column height.
		Subsequent content is filled sequentially into the remaining columns:

		<figure>
			<img src="images/column-balancing-with-figure.svg" alt="Column one contains an image, two and three have content." no-autosize>
			<figcaption>Column height is established by the figure.</figcaption>
		</figure>
	</div>
<wpt title="Tests relating to column balancing in combination with out-of-flow elements.">
column-balancing-with-span-and-oof-001.html
column-balancing-with-span-and-oof-002.html
crashtests/forced-break-in-oof-in-column-balancing-nested.html
crashtests/forced-break-in-oof-in-column-balancing.html
</wpt>

<wpt title="Other tests relating to balancing">
balance-extremely-tall-monolithic-content-crash.html
column-balancing-with-overflow-auto-crash.html
balance-break-avoidance-000.html
balance-break-avoidance-001.html
balance-break-avoidance-002.html
balance-grid-container.html
balance-orphans-widows-000.html
crashtests/balance-with-forced-break.html
crashtests/balancing-flex-item-trailing-margin-freeze.html
crashtests/balancing-tall-borders-freeze.html
</wpt>

<h2 id="overflow">
Overflow</h2>

<h3 id="overflow-inside-multicol-elements">
Overflow Inside Multicol Containers</h3>

	Except for cases where this would cause a column break,
	content that extends outside column boxes
	visibly overflows and is not clipped to the <a>column box</a>.

	Note: See
	[[#column-breaks]] for column breaks
	and
	[[#pagination-and-overflow-outside-multicol]] for whether it is clipped to the multi-column container’s content box.

	<div class=example>
		In this example, the image is wider than the column:

		<figure>
			<img src="images/image-overflow-not-clipped.svg" alt="An imagine in the first column has visible overflow" no-autosize>
			<figcaption>Content visibly overflows and is not clipped to the column box.</figcaption>
		</figure>
	</div>

	<wpt>
	multicol-block-no-clip-001.xht
	multicol-block-no-clip-002.xht
	multicol-clip-001.xht
	multicol-clip-002.xht 
	multicol-clip-scrolled-content-001.html 
	multicol-overflow-clip-auto-sized.html
	multicol-overflow-clip-positioned.html 
	multicol-overflow-clip.html 
	relative-child-overflowing-column-gap.html
	relative-child-overflowing-container.html
	</wpt>


<h3 id="pagination-and-overflow-outside-multicol">
Pagination and Overflow Outside Multicol Containers</h3>

	Content and column rules that extend outside column boxes at
	the edges of the multi-column container are clipped according to the
	'overflow' property.

	A multicol container can have more columns than it has room for due to:

	<ul>
		<li>
			a declaration that constrains the column height
			(e.g., using 'height' or 'max-height').
			In this case, additional column boxes are created in the inline direction
		</li>
		<li>
			the size of the page.
			In this case, additional column boxes are moved to the next page(s).
		</li>
		<li>
			explicit column breaks.
			In this case, additional column boxes are created in the inline direction for continuous contexts
			and additional column boxes are moved to the next fragment(s) for fragmented media.
		<li>
	</ul>

	<wpt>
	multicol-overflow-000.xht
	multicol-overflowing-001.xht
	</wpt>

	<wpt title="Tests to check that a scrollable container isn't split across multiple columns.">
	overflow-unsplittable-001.html
	overflow-unsplittable-002.html
	overflow-unsplittable-003.html
	</wpt>

	Columns that appear outside the multicol container in continuous contexts
	are called <dfn export>overflow columns</dfn>.
	Overflow columns can affect the height of the multicol container.

	<div class="example">
		In this example, the height of the multi-column container has been constrained to a maximum height.
		Also, the style sheet specifies that overflowing content should be visible:

		<pre highlight="css">
		div {
			max-height: 5em;
			overflow: visible;
		}
		</pre>

		As a result, the number of columns is increased.

		<figure>
			<img src="images/height-constraint-overflow-inline.svg" alt="Four columns, one outside the multicol container" no-autosize>
			<figcaption>An overflow column is created in the inline direction.</figcaption>
		</figure>

	</div>

	<div class="example">

		In continuous contexts overflow columns can affect the height of the multicol container.
		In this example a column appears in the overflow which has four lines of text.
		The multicol container is made tall enough to accommodate this column.

		<figure>
			<img src="images/overflow-column-effects-height.svg" alt="Four columns, overflow column is taller than the first three" no-autosize>
			<figcaption>The final column is an overflow column yet is taller than the others.
			The container is tall enough for this column.</figcaption>
		</figure>

	</div>

	<div class="example">
		In fragmented contexts, the overflow content goes into columns in subsequent fragments.
		Given the same content as in example 31
		and a page box that only has room for five lines of formatted text,
		this would appear on the first page:

		<figure>
			<img src="images/pagination-overflow-page1.svg" alt="Three columns" no-autosize>
			<figcaption>The first three paragraphs appear on page one.</figcaption>
		</figure>

		Assuming column balancing, this would appear on the second page:

		<figure>
			<img src="images/pagination-overflow-page2.svg" alt="Three columns" no-autosize>
			<figcaption>The overflow column is moved onto page two.</figcaption>
		</figure>
	</div>

	<div class="example">
		In this example, explicit column breaks are generated after paragraphs:

		<pre highlight="css">
		p {
			break-after: column;
		}
		</pre>

		As a result, the number of columns increases and the extra columns are added in the inline direction:

		<figure>
			<img src="images/height-constraint-column-break-overflow-inline.svg" alt="Four columns, one outside the multicol container" no-autosize>
			<figcaption>An overflow column is created in the inline direction.</figcaption>
		</figure>
	</div>

	<div class="example">
		In [=paged media=], extra columns are shown on the next page.
		Given the same code as the previous example,
		the last paragraph appears on the second page.
		This would appear on the first page:

		<figure>
			<img src="images/pagination-column-break-overflow-page1.svg" alt="Three columns" no-autosize>
			<figcaption>The first three paragraphs appear on page one.</figcaption>
		</figure>

		This would appear on the second page:

		<figure>
			<img src="images/pagination-column-break-overflow-page2.svg" alt="Three columns" no-autosize>
			<figcaption>The overflow column is moved onto page two.</figcaption>
		</figure>

		Due to column balancing, the last paragraph is split across three columns.
	</div>

<h2 class="no-num" id="changes">Appendix B. Changes</h2>

This appendix is <em>informative</em>.

<h3 id="changes-from-multicol1">Changes from the <a href="https://www.w3.org/TR/2024/CR-css-multicol-1-20240516/">Candidate Recommendation (CR) of CSS-MULTICOL-1 of 16 May 2024</a></h3>

<ul>
	<li>Adds a draft of proposed ''column-wrap'' property.</li>
	<li>Adds the ''column-height'' property. <a href="https://github.com/w3c/csswg-drafts/issues/2923#issuecomment-2625930018">Resolved 30 Jan 2025</a>.
	<li>Adds the ''::column'' pseudo-element. <a href="https://github.com/w3c/csswg-drafts/issues/10715#issuecomment-2379924982">Resolved 27 Sep 2024</a>.</li>
	<li>Adds a draft of proposed <css>&lt;integer></css> and <css>auto</css> values for ''column-span''.</li>
</ul>

<h2 class="no-num" id="privacy">Privacy Considerations</h2>

Multicol introduces no new privacy leaks.

<h2 class="no-num" id="security">Security Considerations</h2>

Multicol introduces no new security considerations.

<h2 class="no-num" id="a11y">Accessibility Considerations</h2>

Setting container height and line length can pose challenges for people with visual or cognitive disabilities.
See <a href="https://www.w3.org/WAI/WCAG21/Understanding/reflow.html">WCAG Success Criterion 1.4.10 Reflow</a> and
<a href="https://www.w3.org/WAI/WCAG21/quickref/#visual-presentation">WCAG 1.4.8 Visual Presentation</a> to understand user needs.

<h2 class=no-num id=acknowledgments>
Acknowledgments</h2>

	This document builds upon Håkon Wium Lie's work in [[CSS3-MULTICOL]],
	and is based on several older proposals and comments on older proposals.
	Contributors include:

	Alex Mogilevsky,
	Andy Clarke,
	Anton Prowse,
	Bert Bos,
	Björn Höhrmann,
	Cédric Savarese,
	Chris Lilley,
	Chris Wilson,
	Daniel Glazman and
	Dave Raggett,
	David Hyatt,
	David Singer,
	David Woolley,
	Elika Etemad,
	Giovanni Campagna,
	Ian Hickson.
	Joost de Valk,
	Kevin Lawver,
	L. David Baron,
	Markus Mielke,
	Melinda Grant,
	Michael Day,
	Morten Stenshorne,
	Øyvind Stenhaug,
	Peter Linss,
	Peter-Paul Koch,
	Robert O'Callahan,
	Robert Stevahn,
	Sergey Genkin,
	Shelby Moore,
	Steve Zilles,
	Sylvain Galineau,
	Tantek Çelik,
	Till Halbach

	<wpt hidden>
	crashtests/block-in-inline-become-float.html
	crashtests/multicol-with-oof-in-multicol-with-oof-in-multicol.html
	column-rule-001.html
	column-rule-002.html
	crashtests/text-box-trim-end-and-widows.html
	multicol-span-all-rule-002.html
	parsing/column-span-invalid.html
	</wpt>